Valves and driving circuitries of valves in, for example, anti-lock braking systems (ABS) and Electronic stability program (ESP) systems of vehicles have to fulfill various safety conditions to prevent that a vehicle with such a system might have a defect ABS or ESP system. Some of these safety conditions must be tested on a regular basis, for example, after starting the vehicle and also when the vehicle is driving and the valves of the ABS or ESP system are not actively used. In many systems, the valve driving circuitry may operate in a test mode, which is, for example, activated when the vehicle is switched on and, thus, the ABS or ESP systems are switched on. In the test mode the valve driving circuitry may generate a signal which forces a small current through the mainly inductive load of the valve. The current is small enough to prevent that the valve is controlled in another position (e.g. from open to closed or from closed to open), but the current is large enough to measure parameters of the valve driving circuitry and/or of the valve. The measuring of the parameters is used to decide whether the valve driving circuitry is still operating according to its specification and/or to decide whether the valve still (electrically) operates according to its specifications.
U.S. Pat. No. 5,763,963 discloses a circuit arrangement for driving an inductive load (such as a valve) which comprises an additional measuring device for measuring a current through the inductive load. Parallel to the driving transistor a series arrangement of a transistor and an ohmic measuring resistor is provided and a voltage across the resistor is measured and analyzed to determine the status of the driving circuitry.
In the cited US patent every valve driving circuit must be provided with the additional transistor, resistor and circuit to measure and analyze the voltage across the resistor. When the circuit is being implemented in an integrated circuit, it results in a significant die size increase. When implemented with separate components, a relatively large amount of additional components must be used in the circuits. Therefore, the solution presented in the US patent is relatively expensive. Furthermore, the safety solution of the US patent is not able to detect specific problems in the valve driving system as whole. For example, the valve driving circuit, including the solution of the US patent, may be integrated on one integrated circuit and bonded wires may be used to conduct a current from the integrated circuit (silicon) towards a pin being connected to the valve. Often several bonded wires are used to allow relatively large driving currents. However, the solution of the US patent is not capable of detecting whether one of the bonded wires is not connected and thus, the US patent does not provide a solution for a possible safety risk when one of the bonded wires is not connected.